Water Quality and Growing Marijuana

Rambo December 7, 2011 15
Water Quality and Growing Marijuana

Water is water, right? Wrong. The quality of the water used to grow marijuana can either help your plants thrive or spell disaster for your garden. This is basic stuff you need to know if you want to be successful at growing marijuana.

Water, or H2O, by itself is only that: 2 parts hydrogen and 1 part oxygen. Marijuana plants love the stuff, but we rarely find water that pure. Impurities dissolved or suspended in the water can either help or hinder in growing marijuana. Knowing what’s up with your water and what to do to correct a problem is pretty simple but you need to know the basics first.

TDS and EC

As I mentioned, water is rarely pure. Groundwater picks up minerals as it travels through earth and stone. Bottled drinking water has added minerals for flavor, while municipal water sources often add everything from chlorine to fluoride. Total Dissolved Solids (TDS) is the amount of these solids dissolved in the water or any other solution that can’t be removed with a standard filter.

As you may know, pure water does not conduct electricity. Because of this you can determine how much mineral or organic content is in the water by how well it conducts electricity. Electrical Conductivity (EC) is the measure of a solution’s ability to conduct an electrical current. TDS/EC meters have two electrodes that, when placed in the water or nutrient solution, pass AC voltage between them. The amount of current that passes through the solution indicates the conductivity of the solution. The meter reads this current and converts it to a display that will allow you to either read the EC or TDS (parts per million, ppm) of the water or solution.

By determining the baseline TDS of your fresh water you can later determine the strength of the nutrient solution you are going to mix. For example, if your tap water starts with a TDS of 600 ppm, and your fertilizer of choice suggests a dosage strength of 1200 ppm, you will know that the total TDS should come out to 1800 ppm. If you didn’t know the baseline, you might stop at a TDS of 1200 ppm and, by doing so, give your marijuana plants only half of the required nutrients.

Unfortunately without having your water tested or getting a report from your municipal water supplier, you won’t know the makeup of the minerals dissolved in that baseline TDS. For those growing marijuana indoors, there is an economical solution. Small reverse osmosis systems can remove nearly all of the dissolved solids from your water, bringing the TDS to nearly 0 ppm. You will then need to add back in the trace elements like calcium and magnesium required by marijuana plants, but can do so in the exact amounts needed.

PH Requirement

The pH (potential of Hydrogen) of your water or any other solution is the measure of its acid or alkali levels. When a solution has equal levels of acid and alkali molecules, then the solution is pH neutral. The pH scale runs from 0.0 to 14.0 where 7.0 is neutral, less than 7.0 is acidic, and levels above 7.0 are alkaline or base/basic solutions. Depending on the growing medium used, you want to stay in the slightly acidic range of 5.5 to 6.5. To keep your growing medium and root zone at the correct pH, you need to keep the water or nutrient solution you are using at the correct pH. Contaminants in the water — whether naturally occurring, added by your municipal water supplier, or added when you mix in nutrients and fertilizers – will all affect the pH and may need to be corrected.

You might remember measuring acidity using litmus test strips in high school science or before adding chlorine to a hot tub. While comparing colors can be fun, it’s just not as accurate as you need to be for growing marijuana. To accurately measure the pH of your water or nutrient solution, you are going to need a pH meter.

If your pH doesn’t stay in the safe zone of 5.5 to 6.5, your marijuana plants won’t be able to absorb all of the necessary nutrients, which will lead to nutrient deficiencies. Because nutrient deficiencies can ruin a garden, a quality pH meter is one of the most important tools you will use for growing marijuana. When growing marijuana indoors, you’ll be testing your nutrient reservoir almost every day. If you are growing marijuana outdoors, a pH meter won’t be used as often but is still an essential tool. Luckily pH meters are relatively inexpensive. While options abound, pH meters by Hanna Instruments are the only meters I personally trust.

Water Temperature

When growing marijuana, its best to keep the water temperature about 70 degrees Fahrenheit. Below about 65 the roots have more trouble taking up water and nutrients, while above about 78 can be equally detrimental to the roots. If your nutrient reservoir is located in your indoor grow room, it will probably hold a temperature in this range as it adjusts to the ambient temperature of the room. You should test the water temperature daily with a reservoir thermometer just in case. If you find that the water is staying too cool or that a fresh reservoir takes too long to reach the desired temperature, a reservoir tank heater and thermostat will do the trick. If the reservoir is getting too warm, you have a few solutions. First, if possible, try moving the reservoir out from under the grow light. Just like a water bottle left in the car, the container and water will quickly absorb the heat if left exposed to direct light. Try moving it under a tray table or otherwise creating some shade. If all else fails, try moving the reservoir out of the room entirely.

Aerating Your Reservoir

Roots love oxygen and oxygen-creating movement in water slows the growth of algae. By adding air to your reservoir you can kill two birds with one stone. A small reservoir aeration compressor and air stone similar to what is used in aquariums can be used to aerate the solution of water and fertilizer. This allows you to deliver oxygen to the root zone where it is needed most. Additionally, as the bubbles float to the surface of the tank, the moving air will help stir the reservoir slowing the growth of algae.

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  1. kcplumber January 26, 2012 at 8:11 am -

    First off, I am a master plumber with over 18 of plumbing experience. I see and here alot of talk about ph levels in grow water, especially in hydro growing. From my first grow, I have never used any sort of ph testers. I have also never had problems with my plants absorbing nutrients. The only thing I add to my water, besides nutrients, is Epson salt. Plants grow in soil that have salts in them, and if you are trying to mimic soil conditions with hydro systems its only natural to add salt at the rite levels. The salt will neutralize the acids in water and purify it. Using the same formula that is used for water softner systems, I figured a teaspoon per 8 gallons of water and have amazing results. My growth has been faster and my yield is unbelievable, and this is from my buddy who has been growing since the seventies. He has always been a loyal soil grower, but after viewing my results, he had me help him set up his own system. So quite wasting money on ph testers and spend a $1.50 on a carton of Epson salt. Oh and just so you know, my yield is consistently 16 oz per 3 plants and my entire grow cycle is cut by 2 to 3 weeks. No bs. I grow with bubbleponics. Happy Growing

    • zshaw369 February 20, 2013 at 3:48 pm -

      thats great when you have adjusted water from the city but i run well water with a ph of 8.9 that is way to high and i could not just add eppsom salt and fix it to think so is just wrong, i dont really worry about ph im a soil grower with lots of fungi and bacteria that take care of my ph for the most part but i start with R.O water at a ppm of 0 without my r.o it is 190 out of the tap way to high and city water sucks its got chlorine and chlorimine in it not good for bacteria and fungi for a all organic grower and never use water softeners to get ur water from only if its going straight into a R.O filter if you use it straight way to much salt

      • Rambo March 3, 2013 at 9:29 pm -

        190 ppm is actually pretty low and should not be a problem for growing indoor or outdoor… of course it depends a bit what the 190 is composed of but it shouldn’t be a problem

    • carlos January 28, 2014 at 11:35 pm -

      Congrats I’m from fort worth Texas and I’m a first time grower looking forward to an good harvest tired of spending money when I have a green thumb but don’t know anything about bubbleponics I’m always open to suggestions

  2. Gore January 26, 2012 at 7:21 pm -


    That’s fantastic, I’m pleased to hear you have been blessed with good water. It’s not uncommon for municipal water to be pH adjusted before sending it to consumers. I’m quite sure that a plumber with your experience knows all about municipal water supply, treatment and distribution.

    I however do not have access to any “city” water supply. Most of my life, my water has come from wells, each of which had its own … personality if you will. Some well water is totally unsuitable for hydroponics and requires a great deal of filtration and adjusting before nutrients can be added.

    I think Rambo has had similar experiences with wells and his article reflects this. Water quality varies from well to well and city to city. Additionally it tends to fluctuate seasonally, both in mineral composition and in pH.

    I feel compelled to inform you and other readers that your use of epsom salts absolutely do not work in the manner you described. Water softeners use an ion sensitive resin which bonds to calcium and magnesium. The water passes through a bed of the resin. Negatively-charged resins absorb and bind metal ions, which are positively charged (2RNa(s)+M2+(aq)=R2M(s)+2Na+(aq)(M=Mg/Ca).

    The softeners filtration media does however have a finite capacity to act in this manner. To remove the calcium and magnesium from the media a brine most commonly consisting of NaCL or salt is then passed through the medium, thus restoring a powerful negative ion charge to the resin.

    When you add epsom salt to your reservoir your not neutralizing anything. what your doing is providing the plants with a powerful magnesium supplement. Ironically, because water softeners remove magnesium and calcium and you are supplementing magnesium and sulfur your are in fact performing the opposite of what a softener.

  3. Bob January 26, 2013 at 2:22 pm -

    I been using bottled spring water at 6.4 ph and the cannabis plant as grow big from seed and is grow so fast in its first week I have put organic fert and a few drops of superthrive and cannot belive how quick the little plant has grown it seen to love the spring water. I checked the ph and is at 6.5 iam amazed by the plants progress in the first week and plant is very heathy the first two fan leaves have stood up straight to the flou light iam using before the vegg cycle

  4. ben May 20, 2013 at 6:53 pm -

    so what do you want your ppm in your water to be before/after you add nutrients? some nutrients require your waters tds and ppm numbers to be higher or lower?

    • Rambo May 22, 2013 at 11:27 pm -

      Generally as low as possible. If your water is at 100-200 ppm you are just fine. You may be fine at even higher numbers. You can always drop the ppm to near zero by running it through a reverse osmosis filter.

  5. Skip June 2, 2013 at 7:14 pm -

    I have a question maybe you guys can help with. I’m thinking of buying a fully contained grow cabinet to grow hydroponically. Can I get comments on how good they work and also what nutrients will I need. If anyone has had experience using these, I could use as much advice as possible, Thanks

    • Rambo June 10, 2013 at 10:51 am -

      This is a great question and I am sure the community of growers on this page would be happy to help, however, your questions require an involved answer and are unrelated to this article. Please post your questions again in the forum

  6. Tim October 30, 2013 at 11:36 am -

    I have been using rain water after a long rain
    measures 5ppm and 6.7 ph I adjust to 6.2
    should I aerate after a period of storage?

    • Rambo November 19, 2013 at 5:11 pm -

      Hi Tim,
      Aeration in never a bad idea, it adds oxygen to the water and keeps algae from growing

  7. george June 17, 2014 at 9:25 am -

    I live on a ranch, the water has over 3000 tds. Is there any way i can use this water to grow. I know about reverse osmosis and use it the problem with ro is that when you have this amount tds even a good ro unit will give you very small amounts of good water. \What can i do to be able to use this water?

    • Jim July 11, 2014 at 12:27 pm -

      George – your only option for that water is reverse osmosis with an internal pump for high tds. You might think about rainwater.

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